The invention relates to a coupling dolly for connecting a semi-trailer to an agricultural-type tractor.
The term "agricultural-type tractor" is used here and throughout the remainder of the specification to denote a tractor provided with fittings for the attachment of auxiliary equipment such as agricultural implements or, in industrial applications, material transporting means such as fork lifting apparatus. Such a tractor is distinguished from a road haulage tractor having a connnection for a semi-trailer, such as a fifth wheel connection, mounted above the level of the rear axle of the tractor and is suitable for connection to a semi-trailer by means of a weight transferring coupling hitch for pivotal movement of the semi-trailer about a horizontal pitching axis extending transversely of the tractor below the level of the rear axle of the tractor.
One disadvantage associated with the use of any weight transferring coupling to connect a semi-trailer to an agricultural-type tractor is that the load bearing capacity of the rear axle of the tractor is limited by the manufacturer's specifications. Thus, although a weight transferring coupling can be used to enhance the tractive effort of the tractor, the load carrying capacity of the tractor imposes a limit on the load carried by the semi-trailer.
When coupling a semi-trailer directly to an agricultural-type tractor, it is possible to use a weight transferring coupling which is connected to the tractor by means of first bearing means which allow pivotal movement between the tractor and the coupling about the pitching axis which is disposed below the level of the rear axle of the tractor and this ensures that the horizontal load transmitted from the semi-trailer to the tractor acts so as to press all the wheels of the tractor more firmly on to the supporting surface.
This is important for tractive efficiency and steering control when hauling a heavy semi-trailer by means of a light-weight agricultural-type tractor.
Moreover, in order to allow relative rolling movement between the tractor and the semi-trailer about a longitudinally-extending horizontal rolling axis, so as to prevent overturning forces from being transmitted from the semi-trailer to the tractor, when travelling along a straight-line path, it is possible to use a specialised form of weight transferring coupling in the form of a weight transfer hitch which is provided with second bearing means for rolling articulation between the tractor and the semi-trailer. For constructional reasons, it is preferable to mount these second bearing means at or adjacent third bearing means for yawing articulation between the tractor and the semi-trailer about a vertical axis which must be sited to the rear of the first bearing means to allow pitching articulation between the tractor and the semi-trailer without fouling engagement between the rear of the tractor and the weight transfer hitch.
One disadvantage of such a weight transfer hitch is that, as the longitudinal axis of the tractor turns about the third bearing means on the vertical yawing axis, relative to the longitudinal axis of the semi-trailer, progressively more rolling articulation between the two vehicles is able to take place about the first bearing means on the horizontal pitching axis and progressively more pitching articulation between the vehicles is able to take place about the second bearing means on the rolling axis and this results in a reduction of the rolling stability of the semi-trailer.
Thus, when the tractor and semi-trailer are disposed on a straight-line path, with their longitudinal axes in alignment in a common vertical plane, or parallel in two parallel vertical planes, the weight of the semi-trailer and its load are supported at three points, where the wheels of the semi-trailer bear on the supporting surface and at the second bearing means. These three points define a triangular plane of support which is normally in the shape of an isosceles triangle. Rolling stability can therefore be increased by raising the second bearing means as high as possible so that the height of the centre of gravity of the semi-trailer and its load above the triangular plane of support is as small as possible or, preferably, so that the centre of gravity is disposed below the triangular plane of support. However, as the longitudinal axis of the tractor turns in at least one direction about the third bearing means on the vertical yawing axis, relative to the longitudinal axis of the semi-trailer, the apex of the triangular plane of support moves laterally and downwardly towards the horizontal pitching axis through the first bearing means, causing a two-fold reduction in the rolling stability of the semi-trailer.
This reduction in rolling stability is proportional to the difference in height between the horizontal rolling axis of the second bearing means and the horizontal pitching axis of the first bearing means and, hence, upon the rolling stability of the semi-trailer when the longitudinal axes of the tractor and the semi-trailer are in alignment in a common vertical plane, or parallel in two parallel vertical planes. The reduction in rolling stability of the semi-trailer is also proportional to the lateral movement of the apex of the triangle of support and, hence, upon the distance between the horizontal pitching axis and the second bearing means at or adjacent the third bearing means defining the vertical yawing axis. However, to allow for adequate pitching movement between the semi-trailer and the tractor, it is difficult to reduce this distance between the horizontal pitching axis and the vertical yawing axis by much below 60cm.
Of course, the rolling stability of a semi-trailer could be increased by mounting the semi-trailer axle away from the rear end of the semi-trailer, towards the centre of gravity of the loaded semi-trailer, but this would involve either obstruction or elevation of the load bearing platform to accommodate this axle, and even this improved rolling stability would be reduced during turning, as hereinbefore described. Thus, even though it is possible to increase the rolling stability of the semi-trailer by providing a weight transfer hitch with elevated second bearing means, this involves the use of a weight transfer hitch of very robust and, consequently, expensive construction.
In any case, the increased rolling stability obtained is inevitably reduced when the vertical planes containing the longitudinal axes of the tractor and the semi-trailer are inclined to each other and, if the height of the second bearing means is increased beyond a certain point, this adversely affects the stability of the tractor when the semi-trailer is travelling up or down an incline and the tractor is not longitudinally aligned with the semi-trailer.
It is for this reason that weight transfer hitches such as this have not previously been considered for use on a coupling dolly in the form of a semi-trailer for connecting a load-carrying semi-trailer to an agricultural-type tractor, even though it is known to connect a load-carrying semi-trailer to a tractor by means of a coupling dolly having a conventional fifth-wheel weight transferring coupling with bearing means, which allow pitching rolling and yawing articulation between the tractor and the dolly, arranged together above the level of the rear axle of the tractor so as to ensure that the shape of the support triangle for the coupling dolly remains constant while the loading transferred to the rear axle of the tractor, through the fixed apex of the support triangle, does not exceed the manufacturer's specifications.